#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main()
{
	float a=0, b=0, c=0,s;
	scanf("%f%f%f", a, b, c);
	while (a + b <= c, a + c <= b, b + c <= a)
	{
		scanf("%f%f%f", a, b, c);

	}
	s = (a + b + c) / 2;
	printf("%f",s);

	return 0;

}





